This invention relates to a measuring instrument for use in orthopaedic surgery.
It is important for the success of a surgical procedure to replace an orthopaedic joint to determine as accurately as possible the configuration of the patient's natural joint. This information facilitates the correct selection of the components of a joint prosthesis for use during the procedure, and the correct positioning of the prosthesis components during the procedure. Correct selection and positioning of the prosthesis components can help to ensure that the joint provides the desired biomechanical performance after implantation.
Accurate determination of the configuration of a patient's joint is particularly important in relation to the implantation of a knee joint prosthesis. For example, it can be important to determine (a) the size of the femur measured along the anterior-posterior axis, (b) the proximal extent of the femoral sulcus, and (c) the distance between the femur and the proximal tibia, and the rotational orientation of the tibia relative to the femur, when the joint soft tissue is appropriately tensioned. These factors can determine the selection of components which are to be implanted in the knee replacement procedure, including the size of the femoral component and the size of the spacer which is to be provided between femoral component and the tibial component. The factors can be inter-related in that, for example, a change in the size of the femoral component that is selected will affect the distance between that component and the proximal face of the tibial component. This will have an effect on the tension in the joint soft tissue for a particular size of bearing or spacer component.
It is known to select the size of the femoral component of a knee joint prosthesis by measuring the distance from the posterior condylar surface to the anterior condylar surface. This measurement can be performed before the start of the joint replacement procedure using images of the femur. However, it is generally desirable to measure this distance during the procedure. This can be done through the incision which is made to access the joint by placing the knee in flexion.
The size of the bearing spacer component which is used between the femoral component and the tibial component can also be selected before the start of the joint replacement procedure using images of the joint. Again, it is generally desirable to obtain additional distance data during the procedure. Measurements can be made with the knee in flexion. Additional measurements can be made with the knee in extension. The measurements of the distance between the femur and the tibia requires that the soft tissue extending between the femur and the tibia is placed in tension.
A preferred approach to measuring the femur in a knee replacement procedure makes use of a sizing guide which has a pair of feet which contact the posterior condyles, and a stylus which contacts the anterior cortex, extending beyond the femoral sulcus. The position of the stylus can be adjusted to adjust the point along the length of the anterior cortex at which the anterior cortex is contacted by the stylus. Such a sizing guide is disclosed in WO-A-2011/141722.
US-A-2007/162036 discloses a device which can be used to determine the size of a femoral component and the size of a bearing component for use in knee replacement procedure. The device is located on a rod which is inserted into the intramedullary canal of the patient's femur. The size of the femoral component is measured in a first step by placing a pair of feet in contact with the femoral condyles while the knee is in flexion, and placing a stylus anteriorly in contact with the anterior cortex. In a subsequent step, the plate which carries the pair of feet is moved away from the posterior condyles towards the tibia to place the ligaments spanning the joint in tension. This can then be used to measure the space between the femur and the tibia. This information can be used to ensure that an appropriately sized bearing component is used in the implanted prosthesis, between the femoral and tibial components.